PET systems are used for imaging patients who have received doses of a radiopharmaceutical containing a positron-emitting substance. When a positron from the radiopharmaceutical is captured by an electron, two gamma rays are emitted at 180 degrees from each other. PET systems attempt to reconstruct image information by detecting these gamma rays and storing their coordinates. Multiple detectors are used to detect the two gamma rays in coincidence (in different detectors). Accordingly, PET systems use gamma ray detectors coupled in a coincidence detection mode.
A PET system employing two scintillation detectors is described in a paper presented by Gerd Muehllehner, M. P. Buchin, and J. H. Dudek entitled "Performance Parameters of a Positron Imaging Camera," published in the IEEE Transactions on Nuclear Science, Volume NS-23, No. 1, on February 1976 and also in a paper entitled "Performance Parameters of a Longitudinal Tomographic Positron Imaging System" by Paans, deGraaf, Welleweerd, Vaalburg and Woldring, in Nuclear Instruments and Methods, Volume 192, Nos. 2, 3, on Feb. 1, 1982 pages 491-500. A predecessor of PET systems known as Single Photon Emission Computed Tomography (SPECT) was proposed and developed by Anger in the 1950s.
A problem encountered by PET systems is the unnecessary detection of stray radiation. Radiation may enter the detector from directly under the detector or from outside the area directly under the detector. The area directly under the detector's imaging surface is the field of view, and radiation that comes from outside the field of view is known as stray radiation. Detecting this unnecessary stray radiation results in decreased efficiency of a detector by increasing the detector's count rate while not adding any image information. With an increased count rate, the detector electronics must spend more time counting to form a sufficiently precise image. Thus, the stray radiation increases the detector's dead time.
The efficiency of a single detector may be represented by the ratio R.sub.N /R.sub.S, where R.sub.N represents the non stray count rate and R.sub.S represents the stray count rate. A higher value of R.sub.N /R.sub.S indicates an increased efficiency because of the increase of the non stray count relative to the stray count for the single detector. Ideally, this ratio would be equal to one (100%) through the detector's field of view. In other words, ideally 100% of the radiation detected within the field of view would be non stray radiation. FIG. 1 shows an ideal efficiency curve.
Achieving even a relatively high ratio in actual practice, however, is problematic. The dosage of radiopharmaceutical can be increased so that R.sub.N, the amount of non stray radiation, increases. With an increased R.sub.N /R.sub.S ratio, however, the increase in stray radiation would not be as high as in a system with a lower R.sub.N /R.sub.S ratio, even though the total amount of radiation has increased. In this manner, the imaging could be improved by the higher amount of non stray radiation without as large an increase in stray radiation and the corresponding detector dead time.
In some PET systems having gamma cameras with multiple discrete detectors (i.e., scintillation crystals), septa have been used to assign gamma rays to particular detectors of a gamma camera. Septa are plates made of a material tending to block gamma rays and are designed to block rays coming from certain angles relative to an imaging surface of the detector. Septa in existing systems start at the imaging crystal of the detector and emanate away from the crystal. Septa can potentially cause degradation of parts of the image, however. In particular, parts of the image may be degraded corresponding to parts of the detectors where the septum is placed. For example, in FIG. 3 ray 220 (non stray) is blocked, even though it is within the field of view. It would be advantageous to mitigate cold spots so as to improve image quality.
Hence, it is desirable to improve the efficiency of a gamma camera by reducing the amount of stray radiation detected relative to the amount of non-stray radiation detected. It is further desirable that such efficiency be improved while reducing the occurrence of cold spots.